Polyetherimides (PEIs) are high performance polymers used in a variety of industries such as automotive, aerospace, optics and the like. There are many chemical synthesis processes for preparation of PEIs. In one such process diaryl diether dianhydride, such as bisphenol A dianhydride (BPADA), is polymerized by reaction with a diamine to form PEI. Diaryl diether dianhydride (DA) with predominately 4-ether linkages are sometimes preferred over 3-ether or 3,4-ether linkages since the resulting PEI has lower color properties. Lower color resins have a wider range of commercial uses, e.g., ophthalmic applications. Use of lower color resins also makes it easier to perform color matching applications. Many customers prefer lower color resins, because it enables them to make more transparent parts and develop a wide array of products that meet customer expectations.
Manufacturing processes to produce BPADA typically have an end product containing about 93 percent 4,4-BPADA, about 6.5 percent 3,4-BPADA, and about 0.5 percent 3,3-BPADA. The amount of 3-ether and 3,4-ether linkages is often determined by the purity of intermediates used in manufacturing BPDA. A simplified commercial route to BPADA includes the imidization of phthalic anhydride to N-alkyl phthalimide, which is nitrated to nitro-N-alkylphthalimide (NPI). Following this step is the salt displacement of NPI utilizing an alkali metal bisphenoxide salt to form a diether bisimide. Processes to form the diether bisimide by salt displacement of NPI with an alkali metal bisphenoxide are known can be found in the literature, e.g. U.S. Pat. No. 4,273,712. The diether bisimide can then be converted to a dianhydride as shown, for example, in U.S. Pat. Nos. 4,329,291, 4,329,292, 4,329,496, and 4,340,545.
It has been discovered that the nitration step to manufacture of polyetherimides as described above, produces mixtures containing 4-nitro-N-alkylphthalimide, 3-nitro-N-alkylphthalimide, and proton bearing impurities. Unfortunately, this results in polymers having undesired color properties. The presence of the 3 percent by weight of 3-NPI and 97 percent by weight 4-NPI after nitration above dictates that about 6 percent by weight of the BPADA produced contains the 3,4-isomer.
For the foregoing reasons, there is a need to develop an improved process for isolating purer 4-nitro-N-alkylphthalimide.